Internal combustion engines have been used to produce power and drive machines for over a century. From the beginning, internal combustion engines have undergone many improvements to become more efficient, more powerful, and/or less polluting. Under some conditions, fuel combustion within a combustion chamber can be incomplete, and this can reduce efficiency and produce unwanted emissions of unburned fuel.
Engines can be made more efficient, more powerful, and less polluting with more precise control over the timing for fuel injection, the quantity of fuel injected, and the rate of fuel injection during an injection event. Current state of the art fuel injection valves are hydraulically actuated. For example, a hydraulically actuated valve is actuated by controlling the fluid pressure in a control chamber associated with a valve needle. Typically, hydraulic fluid pressure is kept high to hold the valve needle in a closed position and when hydraulic fluid pressure in the control chamber is reduced, fuel pressure in the nozzle of the fuel injection valve exerts an opening force on a stepped shoulder of the valve needle to lift the valve needle to an open position. In well known fashion, one or more hydraulic fluid control valves can be used in combination with fluid restricting orifices to regulate the pressure in the control chamber by opening or closing hydraulic fluid passages that connect the control chamber to a high pressure supply line or to a low pressure drain line.
State-of-the-art hydraulically actuated fuel injection valves are sometimes referred to as “electro-hydraulic” fuel injection valves because they can employ an electronically controlled hydraulic fluid control valve. For example, solenoid valves that use an electromagnetic actuator can move an armature associated with a valve member from one extreme position to another extreme position, to control the flow of a hydraulic fluid. However, such valves do not permit the valve member to be held in an intermediate position, and, while solenoid valves can be fast acting, they do not allow much control over the speed or acceleration of the valve member when moving from one extreme position to the other extreme position. That is, while solenoid valves are suitable for actuating a control valve between open and closed positions, such valves can not be used to easily control the speed, position and acceleration or deceleration of the valve needle to control the fluid mass flow rate. Some valves are designed with a geometry that allows limited control over the fuel injection rate during a fuel injection event. More recently, control valves actuated by piezoelectric or magnetostrictive actuators (herein referred to collectively as “strain-type” actuators) offer greater control over the hydraulic fluid flow. A strain-type actuator employs an element that changes shape, for example, when a voltage is applied to a piezoelectric element or when a magnetostrictive element is exposed to a magnetic field. Strain-type actuators offer potential improvements over electromagnetic actuators because strain-type actuators can deliver fast and precise movements, while also allowing actuation to positions between the extreme positions and more control over the speed and acceleration of the valve needle. In this way, control valves with strain-type actuators can behave like variable orifices to regulate hydraulic fluid pressure within the control chamber. To modulate injection rate during an injection event, electro-hydraulic valves can employ one or more strain-type actuators to control the flow of the hydraulic actuation fluid and indirectly cause movements of the valve needle by regulating the pressure of the hydraulic fluid in the control chamber. However, even with strain-type actuators operating the control valve(s), electro-hydraulic fuel injection valves still have limitations. Because the valve needle is not directly actuated by displacements caused by the strain type actuator, the speed of actuation is still limited by the speed at which fluid pressure within the control chamber can be adjusted, and variability can be introduced by factors like variations in the differential pressure between the fuel in the nozzle chamber and the hydraulic fluid in the control chamber. Because engines can operate at speeds in the thousands of rpms, the speed of actuation and the precision for injecting the desired quantity of fuel at the desired time can have a significant effect on engine performance.
When the fuel that is injected is a liquid fuel, the fuel can conveniently be used as the hydraulic fluid. However, when the fuel that is injected is a gaseous fuel, an additional disadvantage of electro-hydraulically actuated fuel injection valves is that the hydraulic actuator adds complexity and cost to the fuel injection valve because a hydraulic fluid system separate from the fuel supply system is needed.
“Directly” actuated fuel injection valves are distinguished herein from electro-hydraulically actuated fuel injection valves, in that they employ an actuator that can be activated to produce a displacement that is transmitted directly to a corresponding displacement of the fuel injection valve needle. The displacement produced by the actuator can be reproduced in the valve needle or amplified by transmission elements disposed between the actuator and the valve needle, but with a directly actuated valve, as defined herein, the valve needle displacement is directly proportional to the actuator displacement. With a directly actuated fuel injection valve there is no control chamber as there is with electro-hydraulic fuel injection valves and valve needle movement is governed by the displacement produced by the actuator and not by changing the pressure of a hydraulic fluid. Fuel injection valves can be directly actuated electromagnetically by a solenoid actuator, whereby displacements to an armature produce displacements of a valve needle, but like solenoid actuated control valves employed for electro-hydraulic valves, this type of actuator does not allow much control over the movement of the valve needle. More precise control of the valve needle can be achieved if the fuel injection valve needle is directly actuated by a strain-type actuator. The subject invention is directed specifically to directly actuated fuel injection valves that use a strain-type actuator. Because strain-type actuators can generate high needle actuation forces, the valve can be designed so that variations in the fuel pressure in the nozzle chamber do not have a significant effect on the operation of the fuel injection valve. Furthermore, faster response times are possible because actuation is direct, and does not rely on the flow of hydraulic fluid in and out of a control chamber. Examples of directly actuated fuel injection valves are disclosed in co-owned U.S. Pat. Nos. 6,298,829, 6,564,777, 6,575,138 and 6,584,958.
It is generally believed that rapid closing of the fuel injection valve at the end of an injection event is important for reducing the engine emission levels of unburned fuel, since it is more difficult to combust fuel introduced late in the combustion cycle. In addition, fuel burned later in a piston's power stroke does not generate as much power as the same amount of fuel burned earlier in the power stroke. For example, co-owned U.S. Pat. No. 6,298,829 (the '829 patent) teaches a method of accelerating the closing of a directly actuated fuel injection valve that comprises initially reversing the current to an electrical coil (or the voltage applied to a piezoelectric actuator) to reverse the magnetic field (or the voltage) applied to the coil around a magnetostrictive actuator (or a piezoelectric actuator). To slow down the valve needle and to reduce wear to the valve needle when it impacts the valve seat, the current (or voltage) can be reversed again before being brought to zero. Besides wear, another reason to reduce valve needle speed is that if the closing speed is too high, this can cause the valve needle to bounce off the valve seat, resulting in an uncontrolled amount of late-injected fuel while the valve needle is lifted from the seat on the bounce. A limitation associated with this approach is that the electronic circuits of some actuator drivers are not fast enough to smoothly and rapidly vary the current (for magnetostrictive actuators) or voltage (for piezoelectric actuators) quickly enough to apply this strategy effectively.
The '829 patent also discloses an alternative method whereby the current or voltage is more gradually reduced to zero without reversing the current or voltage. The '829 patent illustrates this method in plots of current and voltage against time by a dotted line with a much shallower slope. While this approach does reduce the impact and wear on the valve needle tip, a disadvantage is that the valve needle is not closed as rapidly, which can result in more fuel being injected later in the combustion cycle.
Desirable attributes of strain-type actuators is their ability to produce very fast movements with high force. While these attributes can help to actuate the valve member rapidly, if the valve member is moving too quickly when approaching the valve seat, in addition to the possibility of bouncing off the valve seat, with a directly actuated fuel injection valve the physical stress from the valve needle impacting the valve seat is transmitted back to the actuator through the transmission elements. Such physical stresses generated from closing the valve can be very sudden if not managed appropriately, and can result in damage to the actuator, manifested by the formation of cracks in the strain-type actuator elements and/or damage or excessive wear to the valve member or the valve seat. Such damage can be caused either by cumulative moderately severe stresses over time or by more severe stresses caused by an isolated abnormally high closing impact. Accordingly, while it is desirable to close the injection valve rapidly, with directly actuated fuel injection valves operated by strain-type actuators there is also a need to reduce the impact force of the valve needle on the valve seat because of the potential for damaging the actuator. The fastest closing time is not optimal if it results in excessive stress on the actuator and other components. Therefore, beyond simply reducing the time for closing a valve, the manner of controlling the acceleration, deceleration, and velocity of the valve needle and the transmission elements between the actuator and the valve needle during the opening and closing movements is important to prevent excessive stress on all of these components.
There is yet another consideration for fuel injection valves that are employed to inject gaseous fuels. Compared to liquid fuels, gaseous fuels do not provide much dampening of the valve needle as it closes. For a fuel injection valve that injects a liquid fuel, the liquid fuel that is squeezed from between the valve member and its valve seat helps to dampen the closing action of the valve needle to further reduce impact forces upon closing. The value of “squeeze films” is well understood in literature on liquid lubrication. A gaseous fuel is defined herein as a fuel that is in the gaseous phase when it is flowing through and out of the injection valve. Accordingly, compared to fuel injection valves intended for liquid fuels, for injecting gaseous fuels there is a greater need for an improved method of closing a fuel injection valve and reducing the impact upon closing. FIG. 8 shows the large variation in fluid density between various liquids and gases. In the case of hydrogen, the density difference approaches two orders of magnitude, versus liquid fuels.